As you see, there is a class definition file, coco_classes.txt under model_data. It defined 80 classes. Unfortunately, shark is not in there.
Training YOLO needs additional efforts, of course. Most time consuming one is to define bounding box to your image files with a class name. Then, you need to change some configuration files and start training. For details, please visit Darknet.
There are several derivatives. PyTorch version, Keras version, etc. One example is, keras-yolo3.
You can also visit at YOLOv3
If you want to see YOLO implementation in this assignment, you will find yad2k/models/keras_darknet19.py and yad2k/models/keras_yolo.py.
In addition, in this community, there is a good summary at Applying YOLO anchor boxes